This invention relates to an axial piston type machine comprising a cylinder barrel mediately or immediately journaled in a casing portion or a part associated with the casing and mounted for axial movement therein. The cylinder barrel has axial cylinder bores. Axial pistons are mounted for sliding movement in the cylinder bores. Connecting passages extend from the cylinder bores and open at an end face of the cylinder barrel. There is a valving surface on the casing portion engaged by said end face of said cylinder barrel due to its axial movability. The valving surface has a pair of arcuate valving recesses adjacent the openings of said connecting passages. These valving recesses are connected to a high pressure connection and a low pressure connection, respectively. There is a stroke plate adjacent the opposite end face of said cylinder barrel. The axis of said stroke plate forms an angle with the axis of the cylinder barrel. The stroke plate is engaged by the axial pistons to produce a reciprocating movement of said axial pistons upon relative movement between cylinder barrel and stroke plate.
In one type of prior art axial piston type machine (which can be either a pump or a motor) the casing portion is a barrel support which is pivotable together with the cylinder barrel. The stroke disc is connected with a machine shaft, and the pistons are linked to the stroke disc. The piston rods are in positive articulated connection to the pistons at one end and to the stroke disc at the other end. The cylinder barrel is, in such arrangement, held centered to a fixed pivot point located on the shaft axis.
In prior art axial piston type machines of this kind, efforts have been made to make the pivot angle, through which the barrel support is pivotable relative to the machine axis, as large as possible, in order to minimize the dimensions of the machine having a given capacity. In prior art machines the centering of the cylinder barrel is achieved by using a concave-spherical end face on the cylinder barrel in abutment with a convex-spherical surface of the barrel support. These also serve as the valving surface for connecting the cylinder bores alternatingly to high and low pressure. There are various problems:
With large pivot angles rather large centrifugal torques are exerted on the cylinder barrel due to the pistons extending more or less into the cylinder bore. On one side of the cylinder barrel the pistons extend only a short distance into the cylinder bore. On the opposite side the pistons extend deep into the cylinder bore. The centrifugal forces acting on the pistons radially outwards thus form a force couple having a level arm equal to the piston stroke. The larger the pivot angle that is used the greater this lever arm becomes. Thus the effective centrifugal torques are increased, and these torques can be substantial.
Further torques occur due to the fact that slideway forces are exerted by the oil pressure due to the piston rods not being parallel to the barrel axis. These slideway forces result in torques which act in the same sense as the centrifugal torques.
Furthermore inertial forces occur with quick changes of the pivot angle.
These torques and forces have to be taken up by the sphericl valving surface, which results in problems.
In another type of axial piston type machine, the cylinder barrel is mounted for rotation in the casing, and the axial pistons slidingly engage a swash plate at one end of the cylinder barrel. The mounting of the cylinder barrel in the casing is usually effected by supporting the cylinder barrel on a shaft. The shaft is rotatably mounted in the casing and means are provided for preventing rotation of the cylinder barrel relative to the shaft. Usually, the axial pistons slidingly engage the swash plate through shoes which are articulated on the pistons by means of ball-and-socket joints.
In axial piston type machines of this kind, the hydraulic forces acting on the pistons are resolved in the plane defined by the centers of the ball-and-socket joints at which the shoes are articulated on the pistons. One force component acts at a right angle to the swash plate and is balanced by a balancing hydraulic pressure area provided below the shoe. Another force component acts in said plane. This latter force component has a radial and tangential component, these components, as acting on a selected piston, being variable during the rotation of the cylinder barrel. In the top and bottom dead center positions of the pistons (the "North" and "South" position), the tangential component becomes zero and the radial component is at a maximum. In the positions angularly spaced therefrom by 90.degree. (the "East" and "West" positions), the radial component becomes zero and the tangential position is at a maximum. The tangential components of the hydraulic forces acting on the plane of the centers of the ball-and-socket joints produce a resultant torque, which becomes effective on the cylinder barrel through the pistons. This torque is produced when the axial piston type machine operates as a motor, i.e. if oil under pressure is supplied to the high pressure side of the machine, and must be exerted when the axial piston type machine operates as a pump in order to deliver oil at a selected pressure. The radial components of the hydraulic forces add up to a resultant radial force which acts on the center of the circle defined by the circular array of centers of the ball-and-socket joints.
In a prior art machine of this type, the cylinder barrel is rigidly supported on a shaft, which, in turn, is mounted for rotation in the casing, whereby this resultant radial force is taken up by the shaft and, through the shaft, by the casing. It is thereby necessary that the cylinder barrel have a fixed orientation in the casing. There is however the further requirement that the cylinder barrel sealingly engage a valve surface on the casing. This is necessary to connect the cylinder bores alternatingly to the high and low pressure side of the machine. These requirements cannot be met merely by high precision manufacturing, particularly since the shaft will deform due to the high hydraulic forces.
Therefore in prior art axial piston type machines the valving means for alternatingly connecting the cylinder bores to the high and low pressure sides of the machine used a disc or the like which is held in the casing for limited movement. This disc is pressed against the end face of the cylinder barrel by plungers or the like, which are exposed to high pressure or to low pressure, respectively. Thereby the disc aligns itself with the end face of the cylinder barrel. In this type of axial piston machine the valving surface is pressed against the cylinder barrel with insufficient balance of the hydraulic forces, because the pulsating force, which acts between valving means and cylinder barrel -- with an odd number of cylinders -- and which tends to separate these parts, must be balanced by a simply constant pressing force. And therefore no complete balancing is possible.
In still another axial piston type machine of this kind, the cylinder barrel is supported for pivotal and limited axial movement in the center of the circle defined by the circular array of centers of the ball-and-socket joints or where the shaft axis intersects the plane defined by this circular array. This can be done by means of spherical gear means. Thereby the support and the shaft take up the radial force described above, with no torques acting on the cylinder barrel. Thus the cylinder barrel is free to align itself with the valving surface of said valving means.
This design, too, suffers from certain disadvantages: Besides the radial forces resulting from the described resolution of hydraulic forces there are other forces acting on the cylinder barrel, for example, forces due to friction or centrifugal forces. These other forces, which, in practice, are not at all negligible produce tilting torques on the cylinder barrel. These tilting torques cannot be taken up by the pivotal support but tend to swing the cylinder barrel away from the valving surface. In addition, this type of prior art machine requires high precision of manufacture in order to make sure that, on one hand, the end face of the cylinder barrel sealingly engages the valving surface and, on the other hand, the cylinder barrel is supported exactly in the plane of the circular array of joint centers, where the resultant radial or lateral force acts. With the large forces involved a small offset of the support point relative to his plane results in rather large tilting torques.
It is an object of the invention to provide an axial piston type machine of the type initially defined, which guarantees, with rather low requirements as to the precision of manufacture of the elements involved, satisfactory taking-up of resultant radial or lateral forces of other torques and also safe and sealing engagement of the cylinder barrel with an abutment or valving surface on the casing or on a casing portion.
In accordance with the invention this object is achieved in an axial piston type machine of the type initially defined in that the cylinder barrel mediately or immediately journaled in said casing portion or said part associated to the casing, respectively, is rotatably supported in two places and an intermediate disc is located between said cylinder barrel and an abutment surface on said casing portion, said intermediate disc contacting said end face of said cylinder barrel adjacent said valving surface, on one side and said abutment surface, on the other side, with spherical contact surfaces curved about different centers of curvature, said abutment surface and said end face being shaped complementary to the respective contact surfaces of said intermediate disc.
In accordance with the invention, the cylinder barrel is supported, on one hand, in two places, while it is, on the other hand, mounted for axial movement in such a way as to maintain the engagement with the end face. By supporting the cylinder barrel in two places, all lateral forces and torques are absorbed by the casing or casing parts, in which the cylinder barrel is mounted. By making the cylinder barrel axially movable, the cylinder barrel will be urged towards the abutment surface on the casing or casing parts by the hydraulic forces acting in axial direction. The alignment problems occurring thereby are solved by means of the intermediate disc having different radii of curvature on both sides, of which one radius may be infinite because the intermediate disc is adapted to align itself in such a manner that there is compensation for misalignments of the other parts.
Advantageously the surface of intermediate disc that contacts the cylinder barrel is shaped to form said valving surface for alternatingly connecting said cylinder bores to high pressure and low pressure, respectively, and the disc is held against rotation relative to said casing portion by a retaining means. The retaining means can be an annular disc located in an annular groove in said casing portion, the annular groove surrounding said abutment surface on said casing portion. The annular disc has four peripheral recesses which are angularly spaced by 90.degree.. There are a first pair of axial projections on said intermediate disc which engage a diametrically opposed pair of recesses on said annular disc. A second pair of projections which are on said casing portion engage the diametrically opposite pair of recesses angularly spaced by 90.degree. relative to said first pair. Such retaining means hold the intermediate disc against rotation about the shaft axis but permit other wise free angular and radial alignment of the intermediate disc with the abutment surface on the casing and with the cylinder barrel.
Preferably said recesses in the annular disc form pairs of parallel planar guide surfaces, between which said projections are guided with corresponding planar side surfaces in a manner to permit limited radial movement.
The intermediate disc is unsymmetrically loaded by hydraulic pressure. This pressure exerts a torque in the intermediate disc tending to tilt it. Compensation for this torque is provided by balancing pressure areas on the circumferential surface of said intermediate disc, said pressure areas communicating with the high and low pressure side, respectively, of the machine.
In one embodiment of the invention the center of curvature of the contact surface of said intermediate disc adjacent said cylinder barrel is in infinity. This arrangement allows short bores to form the conduits connecting the pressure areas to the high pressure and low pressure side of the machine, respectively.
Advantageously said cylinder barrel is mounted for axially sliding movement on the barrel shaft, which in turn is mounted in said casing portion on both sides of said cylinder barrel, said intermediate disc having an aperture through which said barrel shaft extends, there being a gap between said barrel shaft and the wall of said aperture to permit limited movement of said intermediate disc.
The invention is adapted to be used in similar ways and with the same advantage in different general types of axial piston type machines.
One possibility is that said stroke disc is affixed to or integral with a machine shaft mounted in a machine casing, the casing portion forming the barrel support being mounted for pivotal movement relative to said machine casing and stroke disc, and said axial pistons being linked to said stroke disc through piston rods, and further characterized in that said barrel shaft on one hand is pivoted about a fixed break point located on the axis of said machine shaft and on the other hand is supported, with its end projecting through said intermediate disc in said barrel support in a manner permitting axial and pivoting movement.
This has the result that the cylinder barrel is supported through the barrel shaft in two places, namely in the shaft pivot point and in the pivot center of the shaft bearing in the barrel support. Torques acting on the cylinder barrel are absorbed with a large lever arm. This type of mounting, however, avoids the danger of redundancy and compulsive forces. The longitudinal mobility and the intermediate disc guarantee both satisfactory contact between intermediate disc and the end face of the cylinder barrel and satisfactory contact between the intermediate disc and the abutment surface on the casing portion, i.e. in this case the barrel support. Thereby both manufacturing tolerances and slight relative movements resulting from the kinematics of the pivoting movement are compensated for.
In an axial piston type machine of the invention the barrel support may be pivotable about an axis passing through the shaft pivot point. With this type of pivoting of the barrel support, there is, however, a constant, rather large dead volume in the cylinder bores. In order to avoid this dead volume, in prior art machines the barrel support is pivotable about an eccentric pivot axis, whereby in each pivot position a substantially negligible compression space in the cylinder bores is achieved in the top dead center position of the pistons.
In order to achieve these advantages with an axial piston type machine of the present kind while retaining the capability of the parts to align themselves relative to each other without compulsive forces, said barrel support is pivoted about an axis which is at a right angle to the plane containing machine and barrel axes and is spaced from said shaft pivot point, and which, at least approximately, intersects the angle bisector of the axes of said machine shaft and said cylinder barrel, at maximum deflection of said barrel support. Advantageously the distance of said pivot axis from the shaft pivot point is equal to the radius of the circle determined by the pivotal points of said piston rods on said stroke disc. This makes it possible that in one of the pivotal end positions of said barrel support, the center of the spherical contact surface between said intermediate disc and said barrel support is located on the axis of said barrel support.
In axial piston type machines comprising a cylinder barrel mounted in a barrel support, and a stroke disc to which the pistons are positively linked, the cylinder barrel has to be driven by the rotation movement of the stroke disc. The centering of the cylinder barrel about a fixed pivot point located on the axis of the stroke disc, which is achieved with the construction of the invention, permits the cylinder barrel to be driven solely through the pistons and piston rods. Particularly large pivot angles with satisfactory driven connection between stroke disc and cylinder barrel can be achieved, in that said piston rods, at least if subjected to bending load, engage directly the walls of said cylinder bores guiding said pistons of said cylinder barrel, said cylinder barrel being driven by said stroke disc solely through said piston rods.
Preferably each piston rod has generally double-conical shape. A preferred embodiment of the invention is characterized in that each of said piston rods has a recessed portion in the area of the junction of the two cones forming the double-conical shape, there being hollow-conical end faces at each end of said recessed portion, and that a sleeve-shaped piston member is slidably guided in said cylinder bore and has spherical end faces, curved about a common center, in annular contact to said end faces of said recessed portion. A projecting narrow annular surface is formed on the bottom of said recessed portion, said sleeve-shaped piston member being supported on said annular surface midway between its end faces.
In a device of this type the double-cones of the piston rods transmit, in the kinematically effective angular ranges, the rotary movement to the cylinder barrel. By pivotably supporting the sleeve-shaped piston member on the spherical annular surface of the piston rod, the sleeve-shaped piston member guides the piston rod in its central position and transmits slideway pressures to the wall of the cylinder bore. By the abutment of the spherical end faces of the piston member on the end faces of said hollow conical recessed portions, the cylinder space is sealed.
The large pivot angles cause axial forces to be applied to the bearings of the machine shaft in excess of the capacity of roller bearings. Therefore axial bearings provided with hydrostatic balancing pressure areas are used in prior art machines. Such axial bearings, however, suffer from the disadvantage that they are very sensitive to contamination which tend to cause scoring in such bearing.
In accordance with one aspect of this invention an axial hydraulic balancing of the shaft bearings is used to avoid the aforementioned problems. To this end a ring having a spherical end face is mounted on the machine shaft and abuts a shoulder of said machine shaft. The center of curvature of said spherical end face is eccentric with respect to the machine shaft axis. There is also a support disc on the machine shaft. On one side, this disc has a correspondingly spherical end face accommodating said spherical end face of said ring for axially supporting said machine shaft and, on the other side, has a planar end face. Hydraulic balancing pressure areas are formed on both end faces of said support disc. The support disc engages a surface on said casing, which surface is slightly inclined with respect to the axis of the machine shaft.
Due to the small eccentricity of the rotation spherical surface, a slight movement of the support disc on the slightly inclined surface of the casing occurs. This causes friction of movement permitting slow free rotation of the support disc. Thus by the slow rotation of the disc, the abutment is continuously changed similar to the optical process of lapping spherical surfaces. The limiting diameters of the spherical and planar surfaces can be selected in such a manner that no zone remains free from this contacting process. While normally substantially circumferential rotation with slowly varying contact surfaces take place, the support disc can be caused to suddenly rotate at full speed, if there is a contact disturbing the normal running such as might be due to contamination or thermal influences. The sliding movements of the support disc are then also radial due to the inclined abutment, whereby the contamination is dissipated or the local overheating is reduced, the cause thereof being distributed over the whole bearing surface. Thereafter, the original slow rotation will be restored. The relapping process will be finished and the support disc is restored into its original state comprising an amorphous surface.
The invention can, however, be used advantageously in an axial piston type machine of another general type, wherein the cylinder barrel is rotatably mounted relative to a casing or reference member, and wherein the axial pistons slidingly engage a swash plate, which is pivoted in the casing or on the reference member. Preferably said cylinder barrel is supported for solely axial movement on said barrel shaft with the barrel shaft being mounted for rotation in said casing and means being provided for preventing rotational movement of said cylinder barrel relative to said barrel shaft.
In such a construction, a satisfactory mounting of the shaft can be achieved advantageously in that said cylinder barrel has exactly guiding bore portions at both of its ends, said barrel shaft being guided by said bore portions, said means for prevention of rotational movement comprising spline means on said barrel shaft and cylinder barrel between said bore portions.
In order to avoid one-sided mechanical and thermal loading of the pistons in the cylinder bore and to cause a slow rotational movement of the pistons, it is furthermore advantageous that shoes are articulated on said pistons by means of ball-and-socket joints and that said swash plate has a slightly convex surface on the side of said shoes. Thereby the shoe supported on the slightly convex-spherical surface has to execute a permanent small joint movement which results in a slow rotational movement of the pistons.
In order to hydraulically balance the shoes, on the one hand, and to avoid the risk of the shoes lifting off the swash plate on the low pressure side, provision can be made that each of the shoes has a sliding surface provided with a recess forming a central circular balancing pressure area, which recess communicates with the respective cylinder bore through connecting passages in said shoe and said piston; and that said shoes have edge portions and are held in contact with said swash plate by means of an apertured disc extending over said edge portions, said apertured disc having an axially positioned spherical bearing surface which rides on a convex-spherical surface of a spring biased pressing bearing, said convex-spherical surface being curved about the point of intersection of the barrel shaft axis and the plane defined by the centers of the balls of said ball-and-socket joints.
A further advantageous modification of the invention is characterized in that sliding surfaces of said shoes are planar, whereby the planar edge portions of said sliding surfaces and the spherical surface of said swash plate form wedge shaped gaps all around.
With such a construction of the shoes, there is a kind of "aquaplaning", i.e. the shoe is lifted hydrodynamically on the leading side of the respective direction of movement, so that it is easily movable. Thereby, the originally slightly under-dimensioned hydraulic balancing pressure areas below the sliding surface of the shoe is increased by leaking oil under pressure, whereby it substantially balances the force exerted by the piston. On the trailing side of the respective direction of movement, the inner sealing area of the planar side surface around the recess is urged against the swash plate and is thus, at first, subjected to certain wear. Due to this wear, however, the effective area of the hydraulic balancing pressure area will be increased until eventually the pressure area completely balances the piston force and there will be no more wear.